If speculation is not supported by any evidence it is baseless, and I believe I have shown that your speculation is not in fact supported by any evidence.
I contend that it could have come into being the very same moment bestiality itself was first recognized. All it would take is people looking at this new thing and being disgusted by it. As that would be very in line with human nature, to hate that which is different, this is also very much a valid possibility... and one that precludes the possibility that the taboo had any effect on bestiality over time, since it would have then existed at all times bestiality also existed. Now my point is not that this actually happened, but rather that you cannot simply draw conclusions because you want them to be true. We have no evidence as to the existence or strength of the taboo in pre-history, so any speculation is entirely baseless.
Then one can equally conclude that whatever story is being told is equally baseless, including your contention.
While your contention that the taboo "could" have come into existence at the very same moment as the act is a possibility simply because it cannot be proved or disproved, the question becomes "how probable" is it to be the case?
To get an idea of that, let's look to other taboo subjects, like the one I've already mentioned concerning homosexuality in ancient civilizations, and ask yourself how did that work.
In other words, was the soon to be called taboo act more or less prevalent before the taboo versus after?
Let's look at various recreational drugs and ask the same question.
Insest, same question.
Over and over, the action precedes the resultant taboo or law.
So we are to believe that the taboo comes in at the same time as you contend when for such a thing to happen would have it go completely against what we know about other things?
It would make more sense if it happened like every other taboo, with the act preceding the taboo.
So sure it "could" have happened like you say, but really, when all things are considered, how probable is it really?
And let's take what you say is very "inline" with human nature.
It would appear from the examples I gave above, that "inline" with human nature would be again to have the act precede the taboo, or are the examples I give somehow exempt from human nature?
So valid possibility, yes, again because it cannot be proven one way or the other. But "probable"?
I have to think not. Especially considering that someone in enough of a position to pass a law about it is not going to find out about something right away.
Also, remember that humans are very lazy by nature. Sitting down to draft a law because some farmer outside your city walls is boning his goat means someone has to get off his ass and do something. We do things because we want to and sometimes because we need to.
In this case, back to probability, I think it more likely that a law was drafted because it was dipping into his fat pockets, which is a trend that continues even today with our own politicians and corporations.
Put in ancient terms, people were dying from rabies after being bitten by rabid dogs, which decreased production, therefore dipping into pockets. I would imagine at some point the population could have gotten up in arms about it and screamed until a law was passed.
I need to locate it again, but I did find sources trying to link the laws and rabies, and will post when I get a chance to locate it again.
Time and time again over recent and ancient history we see where something new and different was seen as accepted or even good, until something happened to change the way in which it was viewed.
Atomic energy and the friendly atom. Then we had 3 mile island, Chernobyl, Fukushima.
Cocaine in Coca-Cola and other products. We know how that went.
Opiates and Laudanum.
Marijuana.
Alcohol and prohibition.
Good old guns on the table now.
A great example is incest.
How many monarchies practiced it to "preserve their royal bloodline", up until and even after it was proven what inbreeding can lead to?
The way humans react to things of this nature is a base, whether or not you choose to see it as such.